The concentrations of sodium and potassium were measured in seminal plasma of six bulls at monthly intervals between January and May 1979. Mean concentrations (mmol/l) were: sodium, 76.3 plus or minus 25 (range 49.6-102.4); potassium, 49.9 plus or minus 30.9 (14.7-82.6). The mean Na:K ratio was 3.1, range 0.75-7.4. Concentrations differed significantly between bulls and between ejaculates from the same bull.